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  1. #1
    danieletrono's Avatar
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    Dynamic Principles

    How the water ski works?
    According my idea there is a water intake box so a volume of water goes in the impeller and pump which creates an increase of pression at low speed than the ventri principle in the nozzle transforms high prex into high speed and pwc goes....
    Is it true?

    So i can increase speed in two ways......
    - working into the prop impeller
    -woring in the nozzle.

    If i put a small ring increase speed and decrase prex;
    What's about the prop? How type / pitch / number of blade / number of the prop changes this ??

    Thanks


  2. #2

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    [quote=danieletrono;889466]How the water ski works?
    According my idea there is a water intake box so a volume of water goes in the impeller and pump which creates an increase of pression at low speed than the ventri principle in the nozzle transforms high prex into high speed and pwc goes....
    Is it true?

    So i can increase speed in two ways......
    - working into the prop impeller
    -woring in the nozzle.

    If i put a small ring increase speed and decrase prex;
    What's about the prop? How type / pitch / number of blade / number of the prop changes this ?? Sounds good....BUT, you have to have enough volume to propel the ski.

  3. #3
    danieletrono's Avatar
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    So with the change of prop i need to increase volume and prex... With the nozzle transform into speed putting a smaller ring...

    So i need to put a small ring and a 3 blade pitched gaining rpm so increase of volume.

    Is it exact?

    What creates acceleration?

    And the nozzle what does it do? Is it usefull in every case??

    What is the best setup?? A 3 blade prop(which pitch?) + small ring( what is the stock ring for rxpx? Which one could be usefull for a 3 blade?)

  4. #4
    Vern's Avatar
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    Eventual top speed is different than rate of acceleration ... the highest top speed requires different parts than parts optimized for acceleration. Increasing top speed basically requires two things: more thrust and less drag. Less drag generally means a tuned hull bottom, less hull in the water (better ride plate, more ride plate angle, pump wedge, etc), and a less aggressive intake grate. More thrust requires a more efficient jet pump, better impeller, varying nozzle sizes and more hp. If you install a large impeller (15/22 4 blade for example), you will need more hp to turn that impeller to the right rpm (lets just say 8500 rpm for example). If your motor makes enough hp to spin the 15/22 to the rev limiter, you can decrease the nozzle size to lower the rpms and increase thrust and speed due to increased pump pressure. Thrust is a combination of water volume (flow) and pressure ... if you restrict the flow, pressure goes up, but restrict it too much and overall thrust decreases due to less volume. To gain speed, ideally you increase both, but that requires significantly more hp, then experimenting with impellers, nozzles, intake volume, etc.

    Adding speed is a balancing act ... it requires more hp, larger impellers but then fine tuning of the impeller and nozzle size to make the pump the most efficient for your particular motor setup and rpm. Generally speaking, if you have a lot of hp, the biggest impeller and smaller nozzle rings add top speed, while larger nozzles favor acceleration over top speed. There are limits to all of this though, based on the hull, intake cavity size, pump size, available impellers, style of impellers, pump efficiency at certain rpms, etc. A bigger impeller and bigger nozzle can equal the top speed of a setup with a smaller impeller and and smaller nozzle, but the skis may accelerate differently ... there is always more than one way to achieve your goals as far as speed on a PWC.

    Achieving maximum speed on a jetksi is far more complex than on a boat. On a boat, you build the motor to add hp, then just find the best propeller to achieve maximum speed at the rpm your motor needs to be at. They can check efficiency of the hull and propeller by utilizing a slip calcuation that basically measures hull and propeller efficiency. Since there are far more variables with jetskis, the slip factor is not easily measured and there is far more experimenting and fine tuning involved.
    Last edited by Vern; 12-03-2008 at 12:44 PM. Reason: added

  5. #5
    danieletrono's Avatar
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    wonderfull explain...

    So first decide my engine hp....stock 255 ie... Than choice of prop .... Than try different rings...

    Is it correct?

    What is a percet prop for a stock rxpx with 4 air intake ? Which 3 blade? Whuch 4 blade?

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